Power plants in the U.S. produce three-phase alternating current (AC) electricity, which is then distributed, stepped down and/or rectified as needed to produce the desired type of electricity for individual customers. For many applications, it is useful to rectify the AC electricity to generate a direct current (DC) (or near-DC) output. Full-wave rectification of a three-phase input can be accomplished using a six-pulse diode bridge for each output phase.
The six-pulse bridge, however, generates unwanted harmonic distortion that is passed back to the power plant. Existing standards, such as IEEE 519 (Institute of Electrical and Electronics Engineers), govern the amount of distortion that a given component is permitted to cause. For example, IEEE 519 generally limits total harmonic distortion (THD) to about 5%, and the typical six-pulse bridge exceeds that distortion level.
One way of reducing this distortion is to use more than one bridge per phase. For example, some systems use 2, 3 or more bridges per phase, where using more bridges helps reduce the amount of distortion. FIG. 1 shows an example of such a known configuration, in which each output phase uses 6 six-pulse diode bridges, resulting in 36-pulses of current per cycle. As illustrated in FIG. 1, transformer 100 includes an input side 101A and an output side 101B. Three 36-pulse rectifiers 102A,B,C are coupled to secondary windings on the output side of the transformer to produce three output phases. As shown in FIG. 1, each 36-pulse rectifier 102A is fed by six sets of secondary windings 103A. The secondary windings are set at ten degrees separation from one another, and are shown as being +25°, +15°, +5°, −5°, −15° and −25° offset from the primary winding of the input side 101A. The rectifiers in the three phases are all identical in configuration, and are each supplied with six inputs having the same respective phase relationships (e.g., rectifiers 102B and 102C are also fed six inputs that share the same phase relationships found in the six inputs supplied to the first phase rectifier 102A). The result is a three-phase 36-pulse rectification configuration that provides a THD at around 2%, which is well below the IEEE 519 maximum.
FIG. 2 shows one of the 36-pulse rectifiers (102A,B,C) from FIG. 1 in greater detail (as noted above, they are all identical and are provided the same inputs). The 6 six-pulse bridges (201A-F) in each rectifier are shown as being supplied with six inputs from the transformer's secondary windings having the phase relationship noted above (e.g., ten degrees separation from one another). Output is available on positive terminal 202A, common terminal 202B and negative terminal 202C.
The 36-pulse rectifiers used in FIGS. 1 and 2 offer significantly reduced distortion as compared to a single six-pulse rectifier, but at a significantly increased cost. The additional diode bridges and their respective inputs from the secondary windings drive up the overall cost of implementing this configuration. Accordingly, there is a constant need for improved systems that can supply comparable results at a lower cost.